A mixture of (2R,3S)-3-(4-methoxyphenyl)-2,3-epoxypropionamide (1.93 g) and chlorobenzene (39 ml) is refluxed with heating under nitrogen atmosphere. When the reflux is started, a solution of 2-aminothiophenol (1.38 g) and ferric chloride hexahydrate (0.27 mg) in methanol (0.05 ml) is added immediately to the reaction mixture, and the mixture is stirred at the same temperature for 5 minutes to give a reaction mixture containing 3-(2-aminophenylthio)-2-hydroxy-3-(4-methoxyphenyl)propionamide (3.08 g) [(2S,3S)/(2S,3R)=91.3/8.7]. The resulting reaction mixture is concentrated to 16 g. The concentrated mixture is refluxed for 13 hours during which thereto is added methanesulfonic acid (38 mg) in 5 portions (38 mg×5) at intervals of 2-3 hours. The remaining chlorobenzene is evaporated from the mixture, and methanol (10 ml) is added to the residue. The mixture is refluxed for one hour, allowed to cool to room temperature, and further cooled at 8° C. for 40 hours. The precipitated crystals are collected by filtration, washed with chilled methanol, and dried at 50° C. to give (2S,3S)-2,3-dihydro-3-hydroxy-2-(4-methoxyphenyl)-1,5-benzothiazepin-4(5H)-one (2.38 g).
